Screen changers used in the extrusion of molten polymer often incorporate a slide plate located within a housing. The slide plate contains filtering elements for removing impurities from the polymer and is moveable across a bore within the housing. An extrusion apparatus forces polymer through the bore and the filter element which is aligned with it. When the filter becomes clogged with impurities and replacement is desired, the slide plate is translated to a position in which a second filter element is aligned with the bore, and the first filter element can be removed and replaced. Sealing members in the housing bear against the surfaces of the slide plate to prevent polymer leakage from the bore when a filter element is aligned with the bore. Due to high polymer pressure during extrusion, the seal must be capable of being biased against the slide plate with a substantial force to prevent leakage during operation.